Relative Molecular Mass - Example 1

Question:
Given that the formula of a compound is KXO3 and its relative molecular mass is 167. Find the relative atomic mass of element X? (Ar: O = 16; K = 39 )

Answer
Let's say the relative atomic mass of element X = m

The relative moleculaar mass of  KXO3 

= 39 + m + 3(16) = 167
m = 167 - 39 - 3(16) = 80

relative atomic mass of element X = 80